1. Field of the Invention
The present invention relates to a color image forming apparatus of a digital electrophotographic system which includes a laser scanner, and conducts charging exposure and development with respect to a photosensitive medium at a plurality of times so as to form an image by toners of a plurality of colors.
2. Description of the Related Art
Color copying machines of a digital electrophotographic system are known as a recording system which is capable of high-speed printing with high print quality. In this recording system, charging, exposure, and development are carried out with respect to a photosensitive material, and an image is formed on the photosensitive material by toners of a plurality of colors through color-superimposition development and is transferred onto recording paper so as to form a color image.
In printers and copying machines of this type, an area modulation method using a line screen structure, which is generally known in printing technology, is employed as an image structure for reproducing halftone images.
Since algorithms are relatively simple and the cost is low, this method is adopted in many printers and copying machines of the digital electrophotographic system, and a so-called "equiangular line screen" is used in which all the colors of yellow, magenta, cyan, and black are arranged at identical screen angles.
In these full-color image forming apparatuses, writing is generally performed on the basis of a raster scanning method using a laser beam. In this writing, image information written in each scanning is arranged continuously in a slow(sub) scanning direction so as to realize the line screen structure. Accordingly, its screen angle is 90.degree. in a coordinate system in which the fast scanning direction is set to 0.degree.. That is, so-called 90.degree. equiangular line screen, as shown in FIG. 12, is adopted most widely. In addition, in this "90.degree. equiangular line screen, the halftone is reproduced by an image structure in which the thickness of the line with toners superimposed thereon is varied.
Meanwhile, in an image forming apparatus for forming an image on a photosensitive medium by toners of a plurality of colors, when an image of a second or subsequent color is formed, it is necessary to form a latent image by exposing from above the toner image which is previously formed on the photosensitive medium. At this time, since the image formings of second and subsequent colors are affected by the toner image which is previously formed on the photosensitive medium, and an unevenness in potential occurs, which causes the occurrence of unevenness of color. Namely, it becomes difficult for the toner of the second or subsequent color to be developed on the photosensitive medium, so that there occurs deterioration of the image when the image of the second or subsequent color is formed.
As measures against this problem, methods have been adopted wherein exposure conditions at the time of formation of the image of the second or subsequent color are changed with respect to previous exposure conditions.
For instance, Japanese Patent Unexamined Publication No. Sho. 63-65460 discloses a method in which the exposure intensity is strengthened consecutively toward a later step with respect to each step corresponding to each color. Japanese Patent Unexamined Publication No. Sho. 63-66579 discloses a method in which the exposure time per pixel is made consecutively longer toward a later step with respect to each step corresponding to each color. Japanese Patent Unexamined Publication No. Sho. 63-65459 discloses a method in which the beam diameter is made consecutively larger toward a later step with respect to each step corresponding to each color. Japanese Patent Unexamined Publication No. Sho. 63-290676 discloses a method in which the luminous intensity at the time of exposing a second color is controlled on the basis of information on a first gradation and information on a second gradation.
However, in the conventional 90.degree. equiangular line screen, since the screen angles of all the colors are the same, there occurs the deterioration of the image during the formation of images of second and subsequent colors, as described above. Even if the aforementioned method of changing the exposure intensity or time is adopted, in the case of an image pattern in which the state of image formation differs, there arises the fear of an overexposure or an underexposure. In addition, the cost becomes high in introducing hardware for changing the exposure intensity or time.
In addition, in the method in which the luminous intensity at the time of exposing a second color is controlled on the basis of information on first-gradation information and second-gradation information, a page memory is required for storing the first-gradation information, so that the apparatus becomes high.
As a method which is free from problems attributable to the above-described superimposition of toners, Japanese Patent Unexamined Publication No. Sho. 63-144367 discloses a method of forming an image wherein image formation is effected without directly superposing toners of different colors on top of another. However, the image-forming method described in this publication is not intended for image formation using a screen, and the publication describes no specific means for precisely performing positioning for image exposure with the accuracy of not more than one pixel.
In addition, Japanese Patent Unexamined Publication No. Hei. 3-214864 discloses a technique wherein the amount of a reference signal being delayed is changed for each line so as to remove moire. However, the technique described in this publication prevents the occurrence of moire due to the mutual interference of toner images of the respective colors as spacial frequency after the toner images are transferred onto paper, and does not address the problem of the superposition of toner images on the photosensitive medium.